One of the most important steps in building
electronic devices is debugging them to get
them to actually work.
It’s actually uncommon, in my experience,
for my circuits to work the first time,
particularly with “ugly” construction.
This requires a methodical approach, and an
understanding of what voltages to expect in
various parts of a circuit. (The old Heathkit
assembly manuals had tables of expected
voltages on each pin of each tube or transistor.)
Measuring resistance to ground can also
narrow down the problem.
Rarely would it require rebuilding a whole
section of the circuit, unless you had to
remove some parts to get to the faulty one.
While test equipment like a ‘scope and signal
generator can be useful, a common multimeter
and a few tricks can help to narrow down the part
of the circuit that isn’t working. For audio
amplifier stages, for example, touching a finger
to the input of a stage should result on hum and
noise on the output. Working backwards from
the speaker should result in louder noise, until
you reach the bad stage. Simple wideband noise
generators can serve the same function in IF
and RF stages. I used to use the local
oscillator in my general coverage receiver as
a signal generator when needed.
It certainly helps to view the circuit as a set
of individual stages, and have an idea of what
each does. And, when building a receiver,
starting with the audio output and working
backwards, so you can check each stage as
you build it.
Those of us who have built or repaired
equipment will probably have plenty of
stories of perplexing problems that we
had to debug. When I was in high school
I spent my whole summer’s earnings on
a used 75m SSB transceiver as an
incentive to upgrade from Novice, only
to discover that it didn’t work. Lacking
other options, I sat down with a copy of
the RCA Tube Manual to figure out what
voltage to expect on each pin of each
tube. (I didn’t even have a schematic to
start with.)
Finally, I discovered (using my receiver)
that the oscillator wasn’t oscillating. All
the DC voltages looked good, and eventually
I noticed a solder lug on the tuning capacitor
was bent over and shorting to the screw
of a trimmer capacitor. One little tweak
and the set burst into life, after months of
effort.
Meanwhile I had learned so much in the process
that I upgraded from Novice to Advanced in
one sitting.
